1. Field of the Invention
The present invention concerns a method to create an image data set by radial scanning of a raw data space with the use of a magnetic resonance system, wherein unintended time delays of the gradient fields to be generated are corrected. Moreover, the present invention concerns a correspondingly designed magnetic resonance system.
2. Description of the Prior Art
In magnetic resonance tomography, a raw data space (also known as a measurement space or k-space) is typically scanned line by line. However, other scanning patterns are also known. In particular, radial scanning along spokes (i.e. straight lines running through the k-space center) has achieved increasing interest in recent years. Radial scanning offers different advantages, for example a reduced movement sensitivity and the possibility to scan with ultra-short echo times (UTE). Radial scanning is a method from the early days of magnetic resonance, a technique known for a very long time, but previously it could not be widely implemented. The causes of this are primarily inherent technical difficulties that arise upon transition of the scan trajectory along parallel lines to opposite, overlapping spokes.
Time delays of the gradient fields generated in the scanning that lead to a deviation between the assumed and the actual scanned coordinate of the Fourier-transformed data entries (i.e. in k-space) represent a core problem. In conventional, line-by-line scanning, these deviations are irrelevant since all lines are similarly shifted relative to the readout direction. Due to the shift property of the Fourier transformation (shift theorem), this shift leads to a linear phase modulation of the subject in image space with line-by-line scanning. However, since only the magnitude values (and not the phase) are typically considered in the imaging, in line-by-line scanning the phase modulation remains without effect on the presented image.
In radial scanning the gradient delays likewise lead to a linear phase modulation of the scanned components in image space. However, since the readout direction in each spoke differs from the readout direction of another arbitrarily different spoke, a different phase modulation of the contained spatial information respectively results. This different phase modulation leads to image artifacts that are strong in part due to interference effects that significantly reduce the diagnostic value of MR images created with radial scanning.
According to the prior art, no precise and conclusive insights about the physical causes of the gradient delays have existed. The structurally dependent response behavior of gradient coils appears to have a large influence since the observed gradient delays are for the most part anisotropic, meaning that the gradient delay of the gradient coil in the x-direction differs from the gradient delay of the gradient coil in the y-direction. Moreover, the gradient delays depend on the selected readout speed or bandwidth, which on the one hand could indicate an amplitude dependency or delays due to digitization hardware. Finally, there is an influence on eddy current effects and the system adjustment (shim settings).
According to the prior art, essentially two methods for correction of gradient delays are known. In the first method the actual generated gradient fields (and thus the trajectory generated in frequency space) are measured which are then subsequently used for the association of the measurement data in frequency space. For the trajectory measurements that are necessary, according to the prior art the two following documents with regard to the first method are known, which in part use special sensor hardware for the trajectory measurements.
“Simple Correction Method for k-Space Trajectory Deviations in MRI”, J. H. Duyn, Y. Yang, J. A. Frank and J. W. van der Veen, JMR Volume 132, Issue 1, May 1998, Pages 150-153.
“Spiral imaging artifact reduction: A comparison of two k-trajectory measurement methods”, S. M. Fechner, P. T. Sipilä, F. Wiesinger, A. B. Kerr, M. W. Vogel, JMRI Volume 29, Issue 6, Pages 1485-1492.
In the second method the time shift to be expected is estimated and the moment of the dephasing gradient is adapted depending on this such that the actual echo point in time (scanning of the origin position of the frequency space) coincides with the assumed echo time. This method is presented in “Centering the Projection Reconstruction Trajectory Reducing Gradient Delay Errors”; D. C. Peters, J. A. Derbyshire, E. R. McVeigh, Magn Reson Med., July 2003, 50(1): 1-6, wherein the delay for a fixed measurement protocol is determined with a one-time calibration measurement.
In “Robust radial imaging with predetermined isotropic gradient delay correction, P. Speier, F. Trautwein, Proc. Intl. Soc. Mag. Reson. Med. 14 (2006) 2379, the delay to be expected is determined using a linear model so that it is not necessary to conduct a new calibration given a change of the measurement parameters. However, in practice it has been shown that the corresponding correction is not sufficient since the original are apparently system-dependent, adjustment-dependent and also patient-dependent in part. The resulting image quality is therefore not sufficient for a clinical use, in particular not for morphological examinations.